Infrared thermal imaging is commonly used for various applications. For example, Published U.S. patent application Ser. No. 2005/0069180 to Setlak et al. and assigned to the assignee of the present invention, discloses significant advances in infrared imaging for biometric identification of a user's finger. The entire contents of the Setlak et al. application are incorporated herein by reference along with its related applications, Nos. 2005/0089203; 2005/0089202; 2005/0069181; 2005/0063573; 2005/0063572; and 2005/0063571. The infrared imaging described by Setlak et al. includes thermopile sensing pixels that can be used either alone or in combination with other biometric sensing techniques for enhanced accuracy.
Other uses for infrared sensors include automotive, spectroscopic and general imaging application as described, for example, in U.S. Pat. No. 5,689,087 to Jack. In infrared thermal imaging systems typical engineering practice is to minimize the heat flux away from the thermal pickup elements by a variety of thermal isolation methods. The temperature of the thermal sensing elements is allowed to stabilize, hopefully approaching that of the heat source target, and then that temperature is measured. Typical methods of thermally isolating the sensing elements include: back thinning of the semiconductor wafers behind the sensing elements, micromachining void spaces around the sensing elements, floating the sensing elements, etc. Unfortunately, these methods may not be available inexpensively in standard semiconductor foundry processing. These methods may also suffer losses due to the heat conduction across the suspension members and across the measurement connections. The '087 patent to Jack, for example, discloses that the hot junction is thermally isolated from the substrate by being suspended from the substrate on dielectric bridges or, in another embodiment, by a thermally insulating and patterned polymer.
European Published Patent Application No. 1,045,233 A2 discloses an infrared sensor including a flexible diaphragm that may be disposed over a cavity, a thermoelectric element that converts heat to an electrical signal, and an electrothermic element that converts an electrical signal to heat. The thermoelectric element is on the diaphragm and the electrothermic element is on the diaphragm adjacent the thermoelectric element. A reference signal is supplied that is compared with the output of the thermoelectric element for controlling the driving of the electrothermic element. The reference signal and the output signal are compared and the electrothermic element is driven to compensate for the difference between the two signals. Accordingly, the electrothermic element is controlled so as to offset the energy that is detected by the thermoelectric element more rapidly than the thermal response of the diaphragm, thereby eliminating the temperature changes in the diaphragm. The application notes that free of the influence of the thermal time constant of the diaphragm, the response speed with respect to incident infrared light can be greatly improved and the linearity of the thermoelectric conversion can be improved.
Unfortunately, the approach described in the EP 1,045,233 published application requires separate sensing and temperature control elements to be positioned on the diaphragm, and the diaphragm may still use a cavity therebeneath. In addition, many of the conventional approaches to thermal isolation of the sensing elements make use of complicated processing techniques. Such techniques also typically suffer from losses due to the heat conduction across suspension members and across the measurement connections.